Smaller suppliers take an early Direct Debit payment as an account buffer. This is used as credit on your energy account and if your Direct Debit fully covers your bills, you'll get that money back when we flip you away, so please don't worry about being billed too much — it's a normal part of many flips.
Your new will pass meter readings to your previous supplier to close your account. It can take previous suppliers six - eight weeks for final bills to be produced. During this period you may notice payments being collected by both suppliers but please be rest assured you are not paying for energy twice. This is due to OFGEM setting a regulation that the account is to be active until the final bill is produced.
Once your final bill is produced, you will be notified via email. If you are in a credit, this should be refunded directly back to you and if there is a debit, this will be collected as your final Direct Debit payment.
It is through this live Direct Debit that the refund/final payment is taken, however if these double payments will cause you Financial hardship, you are well within your rights to cancel your Direct Debit with your bank, and settle the final bill when it is produced.